1. Field of the Invention
The invention relates to a polyamide composition which is amorphous and transparent, which has reduced flammability, and which is film-free, to a molded article produced from the polyamide composition, and to use of a flame-retardant additive to reduce flammability of polyamides and articles molded therefrom. In particular, the invention relates to use of a phosphonic acid compound as a flame-retardant additive for a polyamide which contains cycloaliphatic monomer units and which is amorphous and transparent.
2. Background of the Related Art
Flame-retarding additives for polymers, particularly polyamides, are well known in the prior art. Such additives include melamine compounds, especially melamine cyanurate, cyanoguanidine, thiourea, magnesium and aluminum hydroxide, ammonium phosphate, halogenated organic compounds, especially brominated organic compounds, in particular in combination with heavy metal salts, and phosphorous by itself and/or in combination with halogen-containing compounds.
These additives are not suited for transparent polyamides and copolyamides, however, since they are deleterious to the transparency of such compositions and articles manufactured therefrom. For example, migration of flame retardant to the surface an article manufactured from such a composition is known to produce a surface film or residue which unacceptably reduces transparency. In addition, the mechanical and physical properties of polyamides are know to change disadvantageously with increasing amounts of additives.
Halogen-free molded articles having reduced flammability and having a VO rating according to Underwriters Laboratories Specification UL-94 for the United States, are of particular interest for the electric and electronic industries. Good transparency of such molded parts represents an important and frequently required quality.
Published European Application No. 0 242 732 A1 provides reduced flammability to transparent molding materials made of amorphous polyamides, primarily of the type PA 6I/6T, by including high proportions of aromatic dicarboxylic acids in the composition. PA 6I/6T represents a polyamide including PA 6I, i.e., hexamethylenediamine and isophthalic acid, and 6T, i.e., hexamethylenediamine and terephthalic acid. Such aromatic dicarboxylic acids include isophthalic acid, terephlhalic acid, and especial polyphosphonates which are themselves synthesized from aromatic phenylene residues and thus may not be safe with respect to health. Despite the relatively high amounts of additives, however, these mixtures often do not reach the VO value for 0.8 mm thick test samples that is essential for industrial uses. Furthermore, with higher amounts of additives, polymer decomposition and reduced transparency become problematic and must be reckoned with. Injection-molded articles made from this prior art mixture have a tendency to be brittle due to the necessarily higher concentrations of aromatic polyphosphonates. This corresponds to a high rate of rejected material when producing thin parts for applications requiring thin parts.
In U.S. Pat. Nos. 3,789,091 and 3,849,368, various ring-shaped phosphonic acid esters are claimed as a flame-retarding additive for various homopolymers. These include phosphonate compounds such as ANTIBLAZE 1045 which is marketed by Albright and Wilson under the tradename ANTIBLAZE 1045. Polyethylene terephthalate, polyacrylonitrile, polyurethane, acetylcellulose, polystyrene and nylon 66 are mentioned as polymers (homopolymers) which may be made flame-retarding using such ring-shaped phosphonic acid esters as flame-retarding additive.
U.S. Patent Nos. 3,789,091 and 3,849,368, however, do not provide any information about amorphous, transparent polyamides which have a special structure and different behavior than the homopolymers discussed. These Patents do mentioned that phosphonic acid esters cause discolorations during the compounding of nylon 66 at temperatures ranging from 280.degree. C. to 310.degree. C. and that, when test samples are ignited, the fire-retardant character is not always found to be self-extinguishing. Frequently, however, this depends to a high degree on the adjustment of the angle of the flame and the positioning of the test sample.
Published European Application No. 0 631 538 B1 (which corresponds to U.S. application Ser. No. 08/002,290 (1993) and to International Publication No. WO 93/18,914) and International Publication No. WO 92/13,717 (which corresponds to U.S. application Ser. No. 08/000,511 (1992)) relate to multi-layered coatings for electrical use, such as especially for cables, made from polyolefins or polyvinychloride and polyamides made flame-retardant with phosphonic acid esters of the type exemplified by ANTIBLAZE 1045. Exemplary polyamides named are primarily semi-crystalline polyamides, such as PA 6, PA 66, PA 610, PA 46, PA 69, PA 11, and PA 12, but mention is also made of an amorphous polyamide composed of certain aromatic diamines. Here too, the special structure and different behavior of amorphous polyamides is not mentioned, however. In particular, no mention is made of migration characteristics of this special class of flame-retardant additives, phosphonic acid esters of the type exemplified by ANTIBLAZE 1045. Also, no information is provided concerning the stability of the flame-retardant behavior as a function of temperature and humidity. However, a deterioration of the yield stress and the elongation at break qualities for nylon 6 made flame-retardant with ANTIBLAZE 1045 is demonstrated and is compared to untreated nylon. The VO rating according to Underwriters Laboratories Specification UL-94 for the United States is obtained definitely only starting with 15 weight % of ANTIBLAZE 1045, that is, only for 0.8 mm thick test samples that drop off faster than thicker test samples.
Mark Huggard of Albright and Wilson (marketer of ANTIBLAZE 1045) delivered a paper titled "New Phosphorus Fire Retardant For Nylon And Polyester Fibers And Other Goods" in 1993 in Atlanta, Ga., U.S.A. The paper reported that the characteristics of nylon 6 made flame-retardant with the phosphonic acid ester ANTIBLAZE 1045 are greatly reduced as a result of the plasticizing effect of this additive so that more moisture is absorbed and that brittleness and yellowing increase strongly during storage at temperatures of, for example, 100.degree. C.
Our own compounding experiments, as well as observations by personnel processing polyamides, have shown that injection-molded parts produced from PA 6 or PA 12 and ANTIBLAZE 1045 sweat very strongly, especially after thermal storage. That is, a greasy coating develops on the surface due to migration of additives to the surface and the injection-molded parts develop yellow/brown discolorations.
From the product brochure for ANTIBLAZE 1045 marketed by Albright and Wilson, it can be seen that the evaporation rate of ANTIBLAZE 1045 at 300.degree. C. is 7% because of the considerable presence of monomeric and dimeric products. Thus, it is recommended that no processing temperatures higher than 250.degree. C. be used to avoid disintegration based on the thermogravimetric analysis curve (TGA curve). When higher temperatures are employed, a conversion to phosphonic acids must be expected as a result of hydrolysis of the phosphonic acid ester.
Based on our own findings, ANTIBLAZE 1045 discolors at 125.degree. C. in 20 days from a transparent, light-colored liquid to a brown and very acidic one. Furthermore, and deviating from experience with polyesters, larger amounts of additives must be compounded in to achieve flame-retarding qualities with semi-crystalline polyamides, e.g., more than 12 weight %, more favorably from 15 to 20 weight %. This factor contributes conspicuously to a deterioration of the mechanical and physical qualities of these semi-crystalline polyamides when made flame-retardant. Further, the reasons why a stronger migration of the ANTIBLAZE 1045 to the surface is noted include, on the one hand, the lower absorption capability of the more compactly arranged semi-crystalline polyamides and, on the other hand, the higher absorption of moisture of the same. As a result of this, a sticky film forms on the surface of tool parts made from such materials, yellowing increases with rising environmental temperatures, and there is generally a loss of the VO character of the tool parts as they become deficient in ANTIBLAZE 1045.
It is therefore an object of the present invention to provide a polyamide composition which is amorphous and transparent, which has reduced flammability, and which has reduced migration of flame-retardant additive so that it is film-free.
It is another object of the present invention to provide a molded article which is molded from the foregoing polyamide composition, which is transparent, which has reduced flammability, and which has reduced migration of flame-retardant additive so that it is film-free.
It is yet another object of the present invention to provide a method of reducing flammability of an amorphous, transparent polyamide and of an article molded therefrom.